U.S. Pat. No. 4,940,677 (Beall et al.) discusses in some detail the extensive research which has been undertaken to devise inorganic glass compositions exhibiting low transformation or transition temperatures (Tg), that characteristic enabling glass batch melting and melt forming operations to be carried out at low temperatures. As was explained there, glasses demonstrating low transition temperatures were known in the glass art, one such family of glasses comprising phosphate-based compositions. Nevertheless, as was also noted there, the chemical durability of phosphate-based glasses was customarily less than that of silicate-based glasses, the durability becoming poorer as the transition temperature of the glass was reduced.
Hence, the primary objective of that patent was to develop glass compositions not only exhibiting a transition temperature below 450.degree. C. and a working temperature below 500.degree. C., but also demonstrating good resistance to attack by water and mild aqueous alkaline solutions. The glasses disclosed consisted essentially, expressed in terms of mole percent on the oxide basis, of at least 65% total of
______________________________________ ZnO 23-55 Na.sub.2 O 0-25 P.sub.2 O.sub.5 28-40 K.sub.2 O 0-25 Li.sub.2 O 0-25 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 10-35 ______________________________________
and up to 35% total of
______________________________________ Al.sub.2 O.sub.3 0-6 ZrO.sub.2 0-5 B.sub.2 O.sub.3 0-8 SiO.sub.2 0-4 Al.sub.2 O.sub.3 + B.sub.2 O.sub.3 0-8 MgO 0-10 Cu.sub.2 O 0-15 CaO 0-10 F 0-5 SrO 0-10 PbO 0-35 BaO 0-12 SnO 0-35 MnO 0-10 PbO + SnO 0-35 MgO + CaO + SrO + 0-15. BaO + MnO ______________________________________
Whereas those glasses did indeed manifest excellent chemical durability, considering the fact that the transition temperatures thereof were below 450.degree. C., research continued to develop glasses having equally low transition temperatures, but wherein the resistance thereof to attack by water and mild aqueous alkaline solutions would be yet improved. That research resulted in U.S. application Ser. No. 07/372,764, filed June 29, 1989 by G. H. Beall et al. under the title RARE EARTH-CONTAINING ZINC PHOSPHATE GLASSES. The glasses described in that application demonstrated working temperatures below 450.degree. C., preferably between 350.degree.-450.degree. C., and resistance to attack by mild aqueous alkaline solutions at least ten times superior to that exhibited by the glasses of U.S. Pat. No. 4,940,677. Those glasses consisted essentially, expressed in terms of mole percent on the oxide basis, of
______________________________________ Li.sub.2 O 0-25 P.sub.2 O.sub.5 28-45 Na.sub.2 O 0-25 Al.sub.2 O.sub.3 0-4 K.sub.2 O 0-25 SnO 0-35 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 10-35 PbO 0-35 ZnO 30-55 SnO + PbO 0-35 ______________________________________
Y.sub.2 O.sub.3 and/or at least one rare earth metal oxide 0.5-5 wherein at least two alkali metal oxides are present.
U.S. Pat. application Ser. No. 07/403,655, filed Sept. 11, 1989 by W. A. Bahn et al. under the title GLASS/GLASS-CERAMIC-PLASTIC ALLOY ARTICLES, describes the preparation of alloys or blends consisting essentially of a glass and/or glass-ceramic and an organic thermoplastic or thermosetting polymer having a working temperature compatible with that of the glass and/or the precursor glass for the glass-ceramic. In forming such alloys or blends, the glass and polymer are combined at the working temperature of the glass to form an intimate mixture. That is, the glass and polymer are in a sufficiently fluid state to permit mixing together to an essentially homogeneous state such that, upon cooling, the resulting body exhibits an essentially uniform, fine-grained microstructure wherein, desirably, there is at least partial miscibility and/or a reaction between the glass and the polymer to provide adhesion and bonding therebetween. The articles produced from that combination of glass and organic polymer demonstrate a unique set of physical properties. For example, at high loadings of glass, i.e., greater than 50% by volume, the articles exhibit high stiffness, high hardness and abrasion resistance, and good mechanical strength imparted by the glass component, coupled with light weight and good toughness and impact resistance imparted by the polymer. The glasses disclosed in U.S. Pat. No. 4,940,677 and U.S. application Ser. No. 07/403,655 have been utilized as the glass component of such alloys.
As was observed above, phosphate-based glass compositions customarily display poorer chemical durability and resistance to attack by moisture than silicate-based glass compositions, and those liabilities become ever more pronounced as the glasses are compounded to demonstrate lower transition temperatures. Therefore, whereas composite bodies composed of organic polymers reinforced through the entrainment of silicate glass fibers and/or particles have long been marketed commerically, glass/plastic composites utilizing phosphate-based glasses have not been marketed to any substantial extent. Thus, although the glass/plastic composite articles known to the art are not porous in a physical sense, the organic polymers are permeable to water; i.e., they are sufficiently permeable to allow water to migrate into the article to thereby come into contact with the glass component. Inasmuch as the glass flakes, fibers, powder, and the like providing the reinforcement for the plastic body present a very high surface area to the incoming moisture, degradation thereof proceeds relatively rapidly. As can be appreciated, that condition becomes more extensive as the proportion of glass in the composite increases and the particle size thereof decreases. Yet, to produce articles exhibiting the stiffness, hardness, abrasion resistance, and mechanical strength imparted thereto through the presence of glass, the glass component must necessarily constitute the greater proportion thereof.
In an effort to broaden the field of organic polymers suitable for forming alloys of the type described in U.S. application Ser. No. 07/403,655,i.e., polymers having lower temperature capabilities and which are customarily less costly, research was conducted to devise glass compositions highly resistant to attack by moisture with working temperatures below about 425.degree. C., preferably no higher than about 400.degree. C., and with transition temperatures no higher than about 350.degree. C., preferably no higher than 325.degree. C.